The combustion of a carbonaceous material such as a solid carbonaceous fuel by reaction with a source of gaseous oxygen is well known. In such a reaction, an amount of air or oxygen equal to or greater than that required for complete combustion is used, whereby the gaseous effluent contains carbon dioxide with little, if any, carbon monoxide. It is also known to carry out the gasification or partial oxidation of solid carbonaceous materials or fuels employing a limited quantity of oxygen or air so as to produce primarily carbon monoxide and hydrogen.
Fuel sources, particularly coals, often have an undesirable halide content. The halogens in halides, such as chlorine in chlorides, form acids in the synthesis gas mixture which can cause severe corrosion in the downstream processing equipment. They also pose environmental and safety hazards if emitted to the atmosphere.
Another problem caused by the halides is reduced efficiency of the process. Formation of some salts in the synthesis gas during processing limits the overall efficiency of the heat recovery from the synthesis gas. This occurs because some salts, such as ammonium chloride, are very corrosive when permitted to condense. Thus, to avoid having the salts condense the synthesis gas cannot be cooled below the sublimation points of various salts. Since the temperature to which the synthesis gas may be cooled is thus limited, the heat recovery from the gas is accordingly limited. Chlorine-containing salts are formed due to the presence of HCl. By removing HCl from the synthesis gas, formation of such salts in the gas stream is reduced or eliminated and the gas can be cooled further to permit more thermal recovery.
A prior known method of removing HCl is by a wet absorption system. In that method the synthesis gas must be cooled and passed through an aqueous absorption column. The HCl is absorbed in the water and neutralized with NaOH. This method has drawbacks since cooling the gas to remove the HCl is inefficient and results in heat/energy loss. Additional equipment and maintenance costs also result from the addition of an absorption column to the process. Economic drawbacks also result from the need for a large water treatment plant due to build up of salts in the water from the absorption column.
It is known from U.S. Pat. No. 5,118,480 to add metals such as Nahcolite to a synthesis gas downstream of the gasifier to remove HCl in conjunction with removing sulfur with a metal oxide sorbent. However, this process lacks the benefits obtained from adding the metal compound to the feed in the gasifier or before the gasifier. The salt formation reaction is believed to benefit from the dissociation and vaporization of the halide and metal compounds in the high temperatures of the gasifier.
It would be advantageous to have a practical and efficient dry method of removing the halides.